The invention relates to a method and a system for the percolation of solid biogenic material in a biogas method having two or more stages. The invention is applied in the area of renewable energy generation.
The production of biogas from renewable raw materials, from biologically available waste and other materials takes place using biogas plants, in which microorganisms transform said materials biochemically into biogas consisting of the main components methane and carbon dioxide.
The transformation. of biodegradable (henceforth “biogenic”) material into biogas takes place in several biochemical steps, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis.
In hydrolysis, water-soluble components are dissolved from the biogenic material, and by a number of extracellular enzymes the non-water-soluble biogenic material is broken down into water-soluble, usually low-molecular, material. To speed up certain decomposition processes, so-called external enzymes may also be used. in the subsequent acidogenesis, the material dissolved during hydrolysis is converted into short-chain organic acids, such as short-chain fatty acids and amino acids. During acetogenesis the organic acids are converted into acetic acid, forming CO2 in the process. The products of the acetogenesis are converted into methane during methanogenesis using methane bacteria.
In single-stage biogas plants, these processes take place in parallel as regards time and space. In two-stage biogas methods, the sub-steps of hydrolysis and acidogenesis (first stage) are separated from the sub-steps of acetogenesis and methanogenesis (second stage) as regards the technical apparatus and processes used. It is thereby possible to separately control the different environmental conditions for the conversion processes taking place respectively during hydrolysis and during methanogenesis This leads to an enhanced level of control and a higher stability of the method. Therefore biogas methods of two or more stages can yield higher methane concentrations in the biogas than single-stage biogas methods. As the hydrolysis process is separated technically, a variety of substrates may be transformed, so that a modular design of the biogas plant is possible.
In current language usage, the first stage of the two-stage biogas method is often simply referred to as the hydrolysis stage, and the second as the methane stage. The hydrolysis stage takes place in the so-called hydrolysis reactor. Methanation takes place in the so-called methane reactor. The aqueous solution leaving hydrolysis is commonly referred to as hydrolysate. In the following, this simplified language usage is adhered to.
In the hydrolysis reactor, the biogenic material is broken down into short-chain organic acids, while hydrolysis gas is formed. Usually, this hydrolysis gas is discharged from the process without being further utilized in the process.
For hydrolysis of solid biogenic material a variety of methods are suitable. Apart from hydrolysis in stirred tank reactors or plug-flow fermenters, methods using percolation are common. In percolation, the solid biogenic material is stacked in hydrolysis reactors, so-called percolators, and sprinkled with liquid (process water). The liquid formed during percolation, which is loaded with organic acids (hydrolysate, herein also referred to as percolate), is transported from the percolators into temporary storage tanks. The hydrolysate, which is stored in a corresponding hydrolysate tank, is fed into the methane reactor in a controlled manner. In this reactor, methane bacteria living under anaerobic conditions form the biogas containing methane. This feeding control makes the control of methane formation possible. According to the present state of knowledge, methane formation takes place in two ways, namely by acetotrophic and hydrogenotrophic transformation, which run in parallel in the methane reactor. The organic matter contained in the hydrolysate is thereby transformed into methane and into further by-products. The remaining liquid, largely freed from organic decomposition products of the biogenic material, is called fermentation liquid. The fermentation liquid is discharged from the methane reactor.
Hydrolysis of solid biogenic material is known from the state of the art and performed using the so-called aerobic percolation (herein also referred to as “open percolation” or “open hydrolysis”), In contrast to anaerobic methods, for which gas-tight percolators are essential, in aerobic percolation there is the possibility of air influx, and therefore the possibility of an aerobic transformation of organic components into carbon dioxide and water, and the possibility of a direct gas discharge into the atmosphere, resulting in losses of the potential biogas formation and in a continuous escape of formed hydrolysis gas. Hydrolysis gas present during aerobic operation contains mainly carbon dioxide and may also contain small amounts of hydrogen, methane, and traces of other gases, such as H2S.
WO 2006/048008 and WO 2007/012328 A1 both describe two-stage biogas methods in which aerobic percolation is performed, so that the hydrolysis gas formed can escape into the atmosphere. The aerobic turnover of biogenic material results in the increased formation of carbon dioxide and water, and therefore the usable energy content of the substrate is disadvantageously reduced.
In addition to the biogas formed by methanation, the hydrolysis gas formed during percolation may partially also contain methane. This occurs especially when the supply of oxygen for percolation is restricted or prevented.
In an open percolation, any methane formed may escape into the atmosphere. This is a disadvantage for both the economic and ecological generation of biogas. It causes an additional pollution of the atmosphere with greenhouse gases and a reduced energy yield, since the corresponding amounts of methane are no longer available for energy recovery. Furthermore, the aerobic breakdown of biogenic material into carbon dioxide and water, which takes place with energy loss for the biogas process, is promoted by oxygen entry during aerobic percolation.
If the percolation is performed in a gas-tight manner and thus any oxygen entry into the percolators is restricted or completely prevented, the organic components are converted in an anaerobic manner. Two-stage biogas methods in which anaerobic percolation takes place are known, for instance, from DE 10 2006 009 165 A1. DE 10 2006 009 165 A1 discloses a method for the two-stage production of biogas from waste containing organic material and a reactor suitable for the operation of this method. The percolator is not ventilated, so that the hydrolysis process control takes place exclusively in an anerobic manner. The hydrolysis gas formed thereby is discharged from the percolators and goes to waste.
By means of the anaerobic conversion of the organic materials used, the methane concentration in the hydrolysis gas can reach higher levels during anaerobic percolation than during aerobic percolation. There are methods known in which methane-forming microorganisms are added to the hydrolysis stage by inoculation, in order to allow production of energetically usable methane already during the hydrolysis stage of aerobic percolation methods.
Further, the conversion into methane can be achieved by means of increased residence time of the hydrolysate in the hydrolysis stage. To this end, DE 10 2008 007 423 A1 discloses a two-stage biogas method and a corresponding system, whereby at least part of the hydrolysis gas is transformed into thermal energy. The thermal energy produced from the hydrolysis gas is used to cover part of the energy requirements arising within the biogas plant. However, this thermal use of hydrolysis gas is disadvantageous if the methane content of the hydrolysis gas is low while at the same time its CO2-content is high, since in this case an inert gas needs to be transported consuming energy.
The increased methane concentration inside the percolator may give rise to ignitable gaseous mixtures, once oxygen is introduced again. In the operation of percolators, the safety implications for this state need to be considered. If, in a gas-tight designed percolator, hydrolysis gas with safety-relevant methane concentrations is present, a safe discharge of the hydrolysis gas is necessary. Moreover, the methane concentration of the gas inside the percolator should be reduced sufficiently, especially prior to emptying the percolators, to be able to rule out an ignitable atmosphere on opening the tanks.
Therefore the gas contained in the percolator (herein also referred to as “gas atmosphere” of the percolator) is usually discharged before opening the percolator by burning it off using gas flares. To this end, in most cases the use of a further source of energy in the form of co-combustion is required, because the sole combustion of the gas is usually not possible.
To avoid the escape of safety-critical concentrations of methane from the gas-tight designed percolators, there are solutions known to reduce the methane concentration in the hydrolysis gas.
EP 1 301 583 B1 discloses a biogas plant designed for single-stage methanation by dry fermentation which distinguishes itself by its superior safety. To achieve this, the plant is equipped with a sensor that measures the partial pressure of oxygen in the fermenter. If the partial pressure of oxygen exceeds a certain limit value, this signals the entry of oxygen through a leak. The biogas pipe is automatically closed, and off-gas which mainly consists of carbon dioxide is fed in from a biogas-using facility. The gases present in the fermenter can escape through a purging valve, so that what remains in the tank in the end is almost exclusively carbon dioxide.
EP 2 103 681 A2 discloses a solution as a further development of the system of EP 1 301 583 B1, in which off-gas containing carbon dioxide from a combined heat-and-power plant (CHP) is used to expel biogas containing methane from a single-stage dry fermentation process. By doing so within a single biogas plant, both fermentation (anaerobic conversion of solid biogenic material into biogas from methane and carbon dioxide) and composting of the previously fermented substrate (aerobic process) can be performed without the necessity of turning the substrate for composting. This method is structured in such a way that in a process of single-stage biogas production, by the end of fermentation a purging of the has phase of the fermented takes place by feeding, in off-gas containing carbon dioxide from a CUP at the end of fermentation process. The methane concentration of the gas present in the fermenter is determined using a sensor. If the methane content of the gas exceeds a certain limit value (at which it makes sense to utilize the gas for energy recovery), the gas is fed into the CHP. If the value is below this limit, the gas is discharged and burned by a gas flare, which may involve feeding. in added fuel. If the methane content of the gas continues to fall below a second, lower, limit value (at which a safe gas discharge from the fermenter is possible), rather than off-gas containing carbon dioxide, fresh air is fed into the tormenter, and simultaneously the gaseous mixture is released into the environment through a biogas exhaust stack. By supplying fresh air it is also possible to run the composting process in the system.
The systems and methods disclosed in EP 1 301 583 B1 and EP 2 103 681 A2 represent single-stage biogas production processes which have the disadvantage that the methane concentration yielded in the biogas is limited. Furthermore, especially in the method of operation disclosed in EP 2 103 681 A2, energy needs to be expended to return CO2-rich off-gas from the CHP to the reactors, thus reducing the overall efficiency of energy production in such a system.
The object of the invention is to make available a method and a system fix obtaining biogas in two stages, in which the gases formed during hydrolysis can be utilized better